Best way to bevel corners for rendering imported CAD models?
As mentioned by Mr Zak, the problem here is your topology which, as is common importing from CAD-Like solid modeling applications, is quite messy and unfriendly for mesh based subdivision modeling.

Ideally you should bevel directly in the CAD application, as it is the most accurate and geometrically correct way to do it, otherwise, cleaning up your mesh may give some positive results.


After importing enter edit mode on your object and press Alt +
J to convert all triangles to quads.
Then add a decimate modifier and set it to Planar, adjust the angle limit to a desired value, not to high as to remove too much detail from your geometry, not to low, as to leave too much geometry for beveling (around 2º worked fine for me)
Add a a bevel modifier and set the Limit Method to Angle